In general, an NOx catalyst configured to clean NOx (nitrogen oxide) contained in exhaust gas is known as an exhaust purifying apparatus located in an exhaust system in an internal combustion engine such as a diesel engine. Various types of NOx catalysts are known. In particular, an NOx catalyst of selective reduction type is well known which continuously reduces and removes NOx by addition of a reducing agent. The reducing agent is commonly used in the form of an aqueous solution of urea. The aqueous solution of urea is ejected and fed from the upstream side of the catalyst. Then, the aqueous solution of urea receives heat from the exhaust and the catalyst and is thus hydrolyzed to generate ammonia. The ammonia reacts with NOx on the NOx catalyst. As a result, NOx is decomposed into N2 and H2O. Such a system configured to continuously reduce and remove NOx by means of the NOx catalyst of selective reduction type using added urea as a reducing agent is called a urea SCR system.
On the other hand, in order to control the amount of reducing agent for example, an NOx sensor is installed downstream of the NOx catalyst to detect the concentration of NOx. The NOx sensor outputs a signal of a magnitude corresponding to the detected NOx concentration. However, temporal changes or the like may cause the output value to deviate gradually from the one obtained when the sensor is new. The deviation may occur particularly in both an offset that is a sensor output value obtained when the NOx concentration is zero and a gain indicative of the degree of an increase in sensor output value which is consistent with the NOx concentration. Hence, the offset and the gain are preferably calibrated at appropriate timings, in order to allow the NOx concentration to be accurately detected even with a deviation in sensor output.
For example, Patent Document 1 discloses that since NOx is not present in the exhaust gas during fuel cut while the supply of fuel to the internal combustion engine is stopped, a reference point for the NOx sensor is learned during the fuel cut.
However, no technique suitable for calibrating the gain of the NOx sensor has been developed, and appropriate measures have been expected to be urgently developed.
Under these circumstances, the present inventors have focused on the NOx sensor's capability of detecting not only the NOx concentration but also an ammonia concentration. The present inventors thus have newly developed a technique to calibrate the gain of the NOx sensor utilizing ammonia obtained from added urea.
The present invention has been made in view of the above-described circumstances. An object of the present invention is to provide an output calibration apparatus and an output calibration method for an NOx sensor which enable the gain of the NOx sensor to be suitably calibrated.